This invention relates to a coating equipment and a method of operating the coating equipment. More particularly, the present invention relates to a coating equipment for producing a zinc-coated steel strip using a hot rolling strip coil as the material and a method of operating the coating equipment.
As described, for example, in "SEITETSU KIKAI SETSUBI SORAN" (Handbook of Iron Manufacturing Machine and Equipment; published on Mar. 25, 1980), in a coating equipment for producing a coated strip with a material of hot-rolling strip coil (which will be sometimes referred to as a "hot coil"), the hot coil is passed sequentially through a coil uncoiler, a strip welder, a pickling line and a heating and deoxidizing furnace, then coated in a coating pot, and thereafter passed through a skin pass mill, etc, to provide a product. The product manufactured by such an equipment is called a "coating strip of hot rolling".
On the other hand, as described, for example, in Japanese Patent Laid-Open No. 122611/1981, a coating strip of cold rolling is produced by an equipment wherein a hot coil is first passed through a pickling line, then through a tandem type cold rolling mill having a differential speed rolling mill at its preceding stage, and thereafter through a coating line for coating strip of cold rolling. In this equipment, the pickling line, the rolling mill, the coating line, etc, are generally constituted as mutually independent equipments, and each of them is equipped with an uncoiler and a coiler for sequentially passing the hot coil through each of these independent lines. According to the technology described in above Japanese Patent Laid-Open No. 122611/1981, a treatment process as independent equipments and its continuing treatment process are selected by providing an uncoiler and a coiler to each of the pickling line, rolling mill and coating line so as to combine them as independent equipments, combining them and controlling the matching between them by a process computer.
Further, Japanese Patent Laid-Open No. 64403/1982 describes equipment in which a hot coil is passed through a descaling apparatus, is rolled by a plurality of passes through a reverse cold rolling mill and is thereafter passed through a cold-rolled material coating line. This equipment is provided with strip storage devices on the entry and exit sides of the reverse cold rolling mill, respectively.
Among the equipments described above, integrated steel makers producing steel products from hot coils mostly use a tandem type cold rolling mill in the rolling line, whereas merely-rolling manufactures who do not produce hot coils by themselves but purchase them from other manufacturers mostly employ a reverse cold rolling mill.
However, the prior art technologies described above are not free from the following problems.
Using the conventional coating equipment for producing hot rolling coating strips, coating strips can be produced at a by far lower cost than that of the coating strip of cold rolling. However, there is a limit of reduction of strip thickness because it is difficult to reduce the finish thickness in hot rolling. In other words, an ordinary hot strip mill is generally a tandem rolling mill having 5 to 7 stands. The minimum practical thickness is 1.2 mm. However, when 1.2 mm-thick strip is produced by a finishing mill having 6 to 7 stands, the surface of the work rolls turns rough due to high pressure, and bending and breakage of the strip occurs during threading and at tailing-off, resulting in extremely low productivity. Although demand for strips of smaller thickness of 1.0 to 0.8 mm exists on the market, this demand has not yet been satisfied.
In the equipment of producing coating strips of cold rolling, the thickness of the product can further be reduced. However, since strip coils must be passed sequentially through independent lines such as a pickling line, rolling line, coating line, and so forth, in each of these lines an uncoiler, a coiler, a welder for joining strips, etc, need to be installed redundantly. Accordingly, the overall installation cost becomes high and the transfer cost of coils between these lines further is added. As a result, the production cost rises. This can be also applied to the prior art technology described in Japanese Patent Laid-Open No. 122611/1981.
As described in "Prior Art" of Japanese Patent Laid-Open No. 64403/1982, the use of a reverse cold mill involves the problems of high costs of installing equipments including strip storing equipments arranged on both entry and exit sides of a reverse cold rolling mill, and of transferring coils between equipments, and further involves the problem that the reverse cold mill cannot inherently roll the leading and trailing edges of a coil. These problems lead to a low yield.
Furthermore, by the prior art technologies described above, it has been difficult to promptly produce a wide variety of coated strips required by users. For example, manufacturers having only a coating line for coating cold-rolled strip cannot purchase timely a variety of coils in small lots. Plating workers are also suffered from this problem. Even in the case of integrated steel makes, the same problem remains unsolved unless close production plans of hot strip mill is made with the sacrifice of the operation factor.